The present invention relates to a colored display on the basis of electroluminescent polymers, having a structured matrix of pixels and a structured second electrode, as well as the fabrication thereof.
The graphic representation of information is constantly gaining in importance in our everyday lives. More and more objects of daily use are being equipped with display elements that permit immediate recall of the information needed on site. In addition to the conventional cathode ray tube (“CRT”), which provides high image resolution but also has the disadvantages of heavy weight and high power consumption, the technology of flat panel displays (“FPDs”) has been developed, in particular for use in mobile electronic equipment. The mobility of the devices places heavy demands on the display to be used. The first to be mentioned is the light weight, which eliminates the conventional CRTs right from the start. Thin depth is another essential criterion. Many devices actually require a display with a depth of less than one millimeter. Because of the limited capacity of the batteries or rechargeable cells in the mobile devices, displays with low power consumption are also required. Another criterion is good readability, even at a wide angle between the display surface and the viewer, as well as readability under various ambient light conditions. The ability to also display multi-color or full-color information is becoming more and more important. And last but not least, the life of the components is naturally also an important prerequisite for use in the various devices. The importance of the individual criteria of demands on the displays is weighted differently depending on the particular area of use.
A number of technologies have already become long established in the market for flat panel displays, not all of which will be discussed here individually. Very dominant today are so-called liquid crystal displays (LCDs). Along with their economy of fabrication, low electric power consumption, light weight and small space requirement, however, the technology of the LCDs also has serious disadvantages. LC displays are not auto-emitting, and therefore can only be read or recognized under especially favorable ambient light conditions. This makes backlighting necessary in most cases, but that in turn multiplies the thickness of the flat screen. Furthermore, the majority of the electric power consumption is then used for the illumination, and higher voltage is needed to operate the lamps or fluorescent tubes. This is produced in most cases from the batteries or rechargeable cells, using “voltage up-converters.” Another disadvantage is the severely limited viewing angle of simple LCDs and the long switching times of individual pixels, which are typically several milliseconds and furthermore are highly temperature-dependent. The delayed screen generation is perceptible and extremely disturbing for example when used in transport systems or video applications.
Other flat panel displays exist besides LCDs, for example vacuum fluorescent displays and inorganic thin film electroluminescent displays. However, these have either not yet reached the necessary level of technical maturity or are of only limited suitability for use in portable electronic equipment because of high operating voltages or manufacturing costs.
Since 1987, displays based on organic light emitting diodes (OLEDs) have been making a name for themselves. These do not have the disadvantages named above. Because of their auto-emission, the need for backlighting is eliminated, which significantly reduces the space requirement and electric power consumption. The switching times are in the range of one microsecond, and are only slightly temperature-dependent, making them usable for video applications. The reading angle is nearly 180°. Polarization films like those needed for LC displays are unnecessary in most cases, so that greater brightness of the display elements can be achieved. Another advantage is the possibility of using flexible and non-planar substrates, as well as simple and economical fabrication.
Two technologies exist for OLEDs, which differ in the nature and processing of the organic materials. In one case low-molecular-weight organic materials such as hydroxyquinoline-aluminum III salt (Alq3) is used, usually applied to the appropriate substrate by thermal vapor deposition. Displays based on this technology are already available commercially, and are utilized at present primarily in automotive electronics. Since the production of these components includes numerous process steps under a high vacuum, however, this technology involves disadvantages due to high investment and maintenance costs, as well as relatively small throughput.
An OLED technology has therefore been developed since 1990 that uses as its organic materials polymers that can be applied to the substrate from a solution, in a wet chemical process. The vacuum steps needed to produce the organic layers are eliminated in this technology. Typical polymers are polyaniline, PEDOT (Bayer), poly(p-phenylene-vinylene), poly(2-methoxy-5-(2′ethyl)-hexyloxy-p-phenylene-vinylene) and polyalkylfluorene, as well as numerous derivatives thereof.
The buildup of layers of organic light-emitting diodes takes place typically as follows: A transparent substrate (glass, for example) is coated over a large surface with a transparent electrode (such as indium tin oxide, ITO). A photolithographic process is then used to structure the transparent electrode according to the use; this later defines the shape of the luminescent pixel.
One or more organic layers consisting of electroluminescent polymers, oligomers, low-molecular-weight compounds (see above) or mixtures thereof are then applied to the substrate with the structured electrode. The polymeric substances are usually applied from the liquid phase by doctoring or spin coating, and recently also by means of a variety of printing techniques. Low-molecular-weight and oligomer substances are usually precipitated from the gaseous phase by vapor deposition or “physical vapor deposition” (PVD). The total thickness of the layers can be between 10 nm and 10 μm, and is typically between 50 and 200 nm.
Onto these organic layers a counter electrode, the cathode, is then applied, usually consisting of a metal, a metal alloy or a thin insulator layer and a thick metal layer. The cathode layers again are usually produced by gas-phase precipitation by thermal vapor deposition, electron beam vapor deposition or sputtering.
The particular challenge in producing structured displays is to structure the buildup of layers described above in such a way that a matrix of individually addressable pixels of differing colors is produced.
In the first step of the OLED production described above, the structuring of the ITO anode, a lithographic technique is available. ITO is extremely insensitive to the typical photoresists and developing fluids, and can easily be etched with acids such as HBr. Structures with a resolution of a few micrometers can be produced without difficulty in this way.
It is significantly more difficult to structure the organic layers and the metal electrode. The reason for this is the sensitivity of the organic materials, which would be severely damaged by the subsequent use of aggressive developing fluids or solvents.
In the case of OLEDS based on low-molecular-weight layers that can be deposited by vapor deposition, the individual functional layers can be vapor-deposited on the substrate in a structured manner by means of a shadow mask, so that red, green and blue pixel areas are produced. For the strip-shaped structuring of the metal cathode (perpendicular to the underlying ITO strips), vapor deposition through a shadow mask is also a possibility. In practice this has significant disadvantages, however, because of the low resolution and the critical alignment of the masks over the substrate.
For that reason, the method of insulating separator ridges was developed for this purpose. In this method, directly after the structuring of the ITO anode, a row of insulating strips with a sharp breaking edge is applied to the substrates, perpendicular to the ITO strips, using a lithographic technique. Following the deposition of the organic layers, the metal cathode is vapor-deposited over a large area, (i.e. without using a shadow mask), with the metal film breaking off at the sharp edges of the separator strips. This forms metal strips that are insulated from one another (lines), perpendicular to the underlying ITO anode (columns). If a potential is applied to a particular ITO anode column and a metal cathode line, the organic emitter layer at the crossing point between the line and the column lights up. These separator strips can have varying cross sections.
In the case of OLEDs based on conjugated polymers which are applied from the liquid phase, the structuring of the individual pixels is substantially more difficult. Conventional techniques such as spin coating or doctoring distribute the polymer solution uniformly over the entire substrate. Subdividing it into red, green and blue areas with a small structural width is therefore difficult, except through subsequent structuring using aggressive lithographic methods, which significantly damage the polymers. For this reason, a number of printing techniques were already used successfully in the past for structured application of polymers. One technique that has especially proven itself here is ink-jet printing, as well as a number of variants thereof. Even with these printing techniques, however, there is great difficulty in preventing the individual closely adjacent color areas from running together. A number of approaches were used in the past to get around this problem.
European patent specification 0 892 028 A2 describes a method in which a layer of an insulating material is first applied to the ITO substrate, in which windows are made at the positions where the pixels are later to be located. This insulating material may be photoresist, for example, which is modified so that it is not wetted by the polymer solutions. The individual drops of the solutions (red, green, blue) are thus enclosed at the appropriate positions without running together, and are thus able to dry there in isolation from one another and produce the polymer layer.
However, this method does not solve the problem of structuring the cathode strips, which have to be applied to the polymer as the last functional layer in passive-matrix driven displays. Various technologies were therefore developed in the past for structuring the cathodes of passive-matrix displays. For monochrome displays, a special method was used to develop separator strips, which are applied first to the structured ITO substrate. The polymer solutions are then spin-applied, one after the other, to these substrates (usually a transport polymer in a polar solution, followed by an emitter polymer in a non-polar solution). As the last layer, the cathode is then vapor-deposited over a large area; it breaks off at the sharp breaking edges of the separator strips and thus forms mutually insulated cathode strips. This method is initially suitable only for large-area application of the polymer solutions, however, and hence not for full-color displays.
As a further development of the method of the separator strips for full-color displays, produced with an ink-jet printing process, it is therefore possible in addition to apply a layer of an insulating material with “windows” (see above). In the process described in European patent 0 951 073 A2, the insulating windows and separator strips are applied to the substrate after the application of individual polymer layers. This is coupled again with the disadvantages of treating the sensitive conjugated polymers with aggressive developer materials, solvents and UV light, already described earlier.
Patent EP 0 732 868 A2 contains a description of a process in which a lithographic treatment of the functional layers is avoided, and at the same time a structured cathode is able to be deposited. To that end, the separator strips for the cathode separation are first produced, and then the functional layers are vapor-deposited in a vacuum through a shadow mask. The serious disadvantage of this method is that the shadow mask does not lie directly on the substrate or the electrode that is on it, but is placed on the separator strips. That greatly intensifies the problem stated earlier of low resolution in the shadow mask technique, because of vapor getting behind the mask.
However, there is a major disadvantage in the window layer for structuring the pixels, in that when the variously colored solutions are applied to the corresponding windows by an ink-jet or micro-metering technique, the solutions can splash or creep into neighboring pixels. This problem is intensified by the fact that with drops having a diameter of some tens of micrometers and velocities of a few meters per second, as used in ink-jet techniques, the kinetic energy of the drops when they strike the substrate is on the same order of magnitude as the surface energy. It is therefore possible, in terms of energy, for the drop to be broken up into many small individual droplets. This problem is especially pronounced when three-dimensional substrates are used, since there the polymer solutions are drawn beneath the breaking edges of the separator strips by capillary forces, where they are able to creep along the separator strips over a range of up to several millimeters.
The problem is made especially acute by the fact that most full-color passive matrix displays use pixels having the same extent in both spatial directions (pv) and (ph), that are made up of the red, green and blue sub-pixels. Since the sub-pixels normally span the entire length of one direction of extension, they become very long and narrow. But at the same time, the display must have a high fill factor F=(pv−3×sv)×(ph−3×sh)/(pv×ph), which indicates the ratio of the functional area to the total area of the pixel. The fill factor determines the brightness of the overall impression of a display. For this reason, the distances between the (sub-)pixels, sv and sh, must be reduced as much as possible. Typical intervals between adjacent sub-pixels are sv=sh=20-30 μm, typical pixel sizes are pv=ph=200-300 μm. This, in turn, of course, significantly increases the danger that the polymer solutions will run into neighboring sub-pixels during printing. In addition, the individual sub-pixels do not have to be of the same size or shape for red, green and blue.
According to the existing art, the electrode applied vertically to the substrate is normally connected as the anode, which actuates the columns of the matrix display, while the horizontally running cathode, which is vapor-deposited after the functional polymer layers are produced, defines the lines of the display matrix. In this case, the separator strips serve only to separate the cathodes, there being pixels of different colors positioned along each individual separator strip (see FIG. 2). As a result, when the variously colored pixels are printed, the polymers creep along the separator strips and are able to get into other pixels.
For conventional passive matrix displays, an operating mode known as “multiplexing” is generally used. In this mode, the lines represent the cathodes of the display, and the columns represent the anodes. The pixel information for each individual line is now written sequentially into the drivers of the anode columns for a short time as data values, and only the driver of the respective cathode line is released. Then after a short time the next line is actuated and the image information for that line is written to the respective driver, etc. Thus each line is only switched on for 1/n of the time, where n is the so-called multiplexing rate (corresponding to the number of lines, in the simplest case). The repetition rate has to be high enough so that the human eye perceives a steady image.